Optical properties of undoped, Eu3+ doped and Li+ co-doped Y2Hf2O7 nanoparticles and polymer nanocomposite films
Desirable phosphors for lighting, scintillation and composite films must have good light absorption property, high concentration quenching, high quantum efficiency, and narrow color emission, etc. In this work, we first show that undoped yttrium hafnate Y2Hf2O7 (YHO) nanoparticles (NPs) display dual blue and red bands after 330 nm light excitation. Based on density functional theory (DFT) calculations, these two emission bands are correlated with the defect states arising in the band-gap region of YHO due to the presence of neutral and charged oxygen defects. Once doped with Eu3+ ions, the YHO NPs show bright red emission, long excited state lifetime and stable color coordinates upon near-UV and X-ray excitations. Concentration quenching is active when Eu3+ doping reaches 10mol% with a critical distance of ~4.43 Å. This phenomenon indicates a high Eu3+ solubility within the YHO host and the absence of Eu3+ clusters. More importantly, the optical performance of the YHOE NPs has been further improved by lithium co-doping. Origin of emission, structural stability, and the role of Li+-co-doping are explored both experimentally and theoretically. DFT calculation results demonstrate that Li+-co-doping increases the covalent character of Eu3+-O-2 bonding in EuO8 polyhedra. Furthermore, the YHOE NPs have been dispersed into polyvinyl alcohol (PVA) to make transparent nanocomposite films, which show strong red emission under excitations at 270 and 393 nm. Overall, we demonstrate that the YHO NPs with Eu3+ and (Eu3+/Li+) doping have high emission intensity and quantum efficiency under UV and X-ray excitations, which make them suitable as phosphors, scintillators and transparent films for lighting, imaging and detection applications.